Technically, the only thing you need to do is renaming the extension from .war to .jar
In addition to that you may require to change some fields in the manifest file due to your requirements. If you supposed to use the resources (ie, Servlets, JSPs....) in your old war file, the best practice is to extract those resources from the war file and use them in the new application. If your new project is an enterprise EJB application (EAR), you can reuse the existing WAR file without doing any changes on it.